The coax cable highway

Last week, two people asked me the same question. My 8-year-old daughter and a guy who I work with basically wanted to know how could a single wire, a single piece of coax cable both provide TV programming into my TV and also carry Internet traffic? How does that work?

Well, instead of thinking it as a single cable or a single channel, think of it as a highway where you can allocate a certain amount of bandwidth for each need that you have. What the cable company does is typically assign 6 MHz of bandwidth for each channel. So you think about this road. The CNN road gets 6 MHz and, you know, USA gets 6 MHz, ESPN gets 6 MHz, you get the idea, specific allocations. So you have separate bandwidth channels within this cable.

Now, if you are also using Internet access with your cable company, what they do is take one of these unused channels and give you that... allocate that bandwidth for you for traffic. But they have 2 different allocations for incoming traffic and outgoing, traffic typically that goes into your PC gets an entire channel 6 MHz. On the other hand, most cable companies were strict. The bandwidth for traffic that comes out of the Internet to 2 MHz. That enables them to control their cost. It also enables them to discourage people from running commercial Web sites onto residential accounts. So that's how they carry the traffic there and this 6 MHz path is usually sufficient for anyone's use for home use on the Internet.

You have to remember though, just because you've got 6 MHz coming from the street into your house, it doesn't mean you're going to get that much when it comes in because even though they have allocated this whole 6 MHz, you're sharing Internet access with everyone on your neighborhood that's riding that infrastructure. So even though this path is pretty wide, your throughput when you actually get in and out of your machine could be considerably less than this if there's a lot of contention in the neighborhood.